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A magnetic disk is a storage device that uses a magnetization process to write, rewrite and access data. It is covered with a magnetic coating and stores data in the form of tracks, spots and sectors. Hard disks, zip disks and floppy disks are common examples of magnetic disks.
A magnetic disk primarily consists of a rotating magnetic surface (called platter) and a mechanical arm that moves over it. Together, they form a “comb”. The mechanical arm is used to read from and write to the disk. The data on a magnetic disk is read and written using a magnetization process.
The platter keeps spinning at high speed while the head of the arm moves across its surface. Since the whole device is hermetically sealed, the head floats on a thin film of air. When a small current is applied to the head, tiny spots on the disk surface are magnetized and data is stored. Vice-versa, a small current could be applied to those tiny spots on the platter when the head needs to read the data.
Data is organized on the disk in the form of tracks and sectors, where tracks are the circular divisions of the disk. Tracks are further divided into sectors that contain blocks of data. All read and write operations on the magnetic disk are performed on the sectors. The floating heads require very precise control to read/write data due to the proximity of the tracks.
Early devices lacked the precision of modern ones and allowed for just a certain number of tracks to be placed in each disk. Greater precision of the heads allowed for a much greater number of tracks to be closely packed together in subsequent devices. Together with the invention of RAID (redundant array of inexpensive disks), a technology that combines multiple disk drives, the storage capacity of later devices increased year after year.
Magnetic disks have traditionally been used as secondary storage devices in computers, and represented the mainstream technology for decades. With the advent of solid-state drives (SSDs), magnetic disks are no longer considered the only option, but are still commonly used.
The first magnetic hard drive built by IBM in 1956 was a large machine consisting of 50 21-inch (53-cm) disks. Despite its size, it could store just 5 megabytes of data. Since then, magnetic disks have increased their storage capacities many times-folds, while their size has decreased comparably.
The size of modern hard disks is just about 3.5 inches (approx. 9 cm) with their capacity easily reaching one or more terabytes. A similar fate happened to floppy disks, which shrunk from the original 8 inches of the late 60s, to the much smaller 3.5 inches of the early 90s. However, floppy disks have eventually became obsolete after the introduction of CD-ROMs in the late 1990s and now have all but completely disappeared.